Posted
by
samzenpuson Sunday April 01, 2012 @03:00PM
from the where-are-we? dept.

Jimme Blue writes "The use of pulsars as a GPS analogue holds the promise of fixing a spacecraft's location to within 5 km, anywhere in the galaxy. While not ready for immediate use, it may be ready for use within the Solar System in the next 10-15 years. From the article: '"The principle is so simple that it will definitely have applications," said Prof Werner Becker from the Max-Planck Institute for Extraterrestrial Physics in Garching.
"These pulsars are everywhere in the Universe and their flashing is so predictable that it makes such an approach really straightforward," he told BBC News.'"

I assume you took this prior art as inspiration? [wikipedia.org] (Incidentally, I also tend to recall Sagan mentioning pulsars in the first episode of Cosmos being once believed to be a form of alien navigation. Guess that's why he and Drake put them on the plaque.)

Tony Hewish, who got the Nobel Prize partly for his part in the discovery of radio pulsars, took out a patent on this idea not long after the discover of pulsars in 1968. I suspect the patent has now expired, but it's by no means a new idea.

They were. A pulsar signal is a repeating pulse - a characteristic previously thought to be unique to artificially generated transmissions, perhaps a beacon of some purpose. It must have been quite a letdown when someone worked out a natural process that could generate such a signal.

Within the solar system, visibility to a known set of pulsars shouldn't be an issue, but as you venture outside the solar system, which pulsars are visible may begin to change as pulsars don't emit in all directions. In practice, most pulsars in a given galaxy probably rotate/emit more or less in the galactic plane, so, even within a galaxy, it's probably a good reference. But that's definitely a risky method if you start moving out of the galactic plane.

They might underestimate our military capacity, though, if they see what passes for entertainment around here.
The ensuing moment of surprise should give us enough time to take the most pressing action, i.e., change our FB status to dead.

Your logic is flawed. The Milky Way doesn't follow lie along the equator because the earth is tilted on it's axis.

The fact that we can see other pulsars (many of them) from within the Milky Way means that those pulsars are either rotating on an axis nearly perpendicular to the galactic plane (aka galactic equator), or they're tilted relative to to the galactic plane and we happen to be nearly perpendicular to their axis of rotation. Since we know that not all stars rotate in the galactic plane (e.g. the solar systems ecliptic plane is inclined about 60 degrees from the galactic plane, and we have now observed enough other planetary systems to know that isn't uncommon), then it's likely that some of the pulsars we can see are spinning approximately in the galactic plane, and that some are not. But given there are many more random orientations that are not on an axis perpendicular to the galactic plane, it's probable that most of the ones we can observe from earth are nearly perpendicular to the galactic plane, and will thus be visible throughout most of the galaxy. As we travel around the galaxy within the vicinity of the galactic plane, the pulsars we can see from any given point will be those that spin on an axis nearly perpendicular to the galactic plane, plus some smaller number at other orientations that happen to have an axis of rotation approximately perpendicular to a line between that point and the pulsar.

The Earth is tilted on its axis by 23 degrees, as you later point out it is tilted at 60 degrees compared to the galactic ecliptic. These do not match. As far as I can tell the orientation of a star's rotational axis is random, and probably uniformly distributed.

Assuming a pulsar's magnetic field is roughly aligned with its rotational axis, we would not even see distant pulsars in this galaxy if their axis was perpendicular to the plane of the galaxy.

The earth's tilt and the solar ecliptic plane don't have anything to do with our ability to view pulsars in/near the galactic plane. What would affect it is the solar system's distance from the galactic plane, and by galactic standards, that's pretty small.

What is relevant is the axis of rotation of the pulsar, the angular height of it's beam, it's distance from us, and it's distance from the galactic plane (and any intervening matter than might obscure it). Again, what's probable is that most of the ones we see have an axis of rotation approximately perpendicular to the galactic plane, such that their beam is "approximately in the galactic plane", giving visibility to that pulsar from most stars near the galactic plane. The larger the angular height of the beam, and the more distant the pulsar, the farther outside a perfect "perpendicular line of sight" the pulsar will be visible.

The earth's tilt doesn't matter, it is just at 23 degrees fairly small and I was ignoring it. It is the rather the sun's that matters, as it is just our closest example of a star. The sun doesn't rotate in the plane of the galaxy, and there is no reason to think that other stars preferentially do, either. Since a pulsar is created in a supernova there is even another randomizing event involved. A pulsars axis is not going to be governed by the galaxy's plane.

No, but a pulsar's axis of rotation will be based primarily upon it's progenitor star's axis. The blast may alter it somewhat, but the angular momentum of the star and it's axis of spin are likely to survive largely as they were prior to the SN.

And as I stated before, our solar ecliptic plane's angle to the galactic plane has ZERO effect on the pulsar's we can see. Nothing at all.

What matters are exactly the items in my previous post (that you seem to have ignored).

As the spaceship moves some pulsars will drop out of visibility and others will become visible - so the spacecraft can lock on to the new ones, using the old ones to calibrate them. That would work unless we develop some kind of 'jump' technology where we appear at great distance from our last position and have no pulsars in common between the two places, but that would be an interesting problem to have.

You are assuming that most pulsars are in the Galactic disc. However, pulsars are born with kick velocities that can cause them to move far out into the Galactic halo. The true distribution of pulsars in the Galaxy is not known, but it almost certainly does not follow the distribution of stars. Now, because pulsars have limited lifetimes, they are probably much more common near the Galactic disc than far away from it, but this has nothing to do with their rotation axes.

Most pulsars are within a couple kpc of the galactic equator, "close enough" be astronomical measurements. The angular displacement from a distant source that is still small enough to be geometrically "near the equator"

The same could be said for the whole of the Milky Way! It's a disc, not a diffuse cloud (or elliptical galaxy), and so most of the stars are concentrated about the equator. That you would also find most known pulsars there is interesting, but hardly surprising.

The problem isn't visibility, the problem is carrying an antenna of sufficient sensitivity and angular precision.

But the thing you really need to know to navigate a spacecraft isn't position - it's velocity, as you're position is changing every second. While you can determine velocity from a series of positions, any errors in position will propagate into an error in velocity.

Only if the errors amount changes per sample. It should however remain consistent unless your speed changes, so while you may be off by some distance, you should be consistently off by that distance so your speed would still be correct.

It doesn't prevent it from working (inside or outside the galaxy), but orientation does affect visibility. The problem isn't whether this can work, the problem is a lack of knowledge about visibility, and that's a problem we can't completely answer from Earth. Therefore, we won't know which pulsars are visible from a given location until we actually travel there (or at least have traveled far enough to map out the probable visibility of a sufficiently large number of pulsars). We may be able to determine th

This has been in use in sci-fi since the dawn of space opera. It gained sufficient use that it was internalized to the point that it's rarely mentioned anymore, you could even say it's why most sci-fi expects a reliable knowledge of location and date even in the face of miss-folds and unplanned time travel.

It's also largely useless. Knowing where you are to within 5km relative to a star and some planets is useful when you are within a star system. Knowing where you are in some arbitrary coordinate system but not knowing where any of the nearby planets are in that coordinate system is a recipe for a '60s TV series.

True, but there are lots of ways to work. For instance send a few probes out in different directions at (say) 1% of lightspeed with decent telescopes.

After a few decades the probes (which can locate themselves using pulsars) can get a fantastic parallax baseline to pin down the 3D location of anything within a few hundred lightyears very accurately indeed. Then you can use pulsars to steer the next generation (faster) probe through the target solar system.

I think its awesome that while we had to build a GPS constellation for earth, the Universe has naturally provided a system usable for precision guidance for interstellar travel. Science is fucking awesome.

5 km should be trivial. Less than 10 meters should be doable using consumer grade hardware.

Pulsar emission areas have been mapped to about 2 meters according to some research out of Australia using radio telescopes. If they can map an emission source to 2 meters several light years away, then I'm thinking they should be able to get positions better than 10 meters when combining things. Another way to look at is that 2 m is about 6 ft which is about 6 nano-light-seconds. The trick is to adjust a clock tic

So, all these pulsars (like everything else in the universe) are always in motion... That motion isn't constant and the direction is also subject to change. In addition to their rate, wouldn't you also have to know their velocity/vector at the time of the pulse?

So, all these pulsars (like everything else in the universe) are always in motion...

It's hard, but that is why there is that lousy accuracy of 5-10 km.

5-10km isn't all that lousy, when you're trying to find your position within light-seconds of the nearest major celestial body. This would give you about 3/10000, or 0.0000003% margin of error to avoid running into that star/planet/signularity
For avoiding space-dust and the like you wouldn't need this (you could use radar, or something similar)

How do we know a pulsar's period cannot change over millenia? I mean all sorts of things can change a pulsars period.. collision with a red dwarf for one.

Isn't it easier, and far more accurate to use the regular stars? There are billions of them.. many of which have known rotational periods, brightness variability, and proper motions that can be detected via doppler shifts and other means. The Hipparchos satellite produced a fairly accurate 3D map of the neighborhood.. that can be a good starting point. Every star has it's own spectrographic and brightness variation signature.. Sure black swan events may change a stars spectrum, variability cycle, and other things.. but there are a billions stars.. a spacecraft can navigate by tracking just a few million of them (a wide field gigapixel camera and few spectrographic telescopes, should be all it needs).. its extremely unlikely that more than a few percent of them will change enough to cause navigational errors.. just update the star tables every 10,000 years.. normal GPS has to it that way more often with satellites.

When i said update the star tables every 10,000 years.. i meant the spacecraft itself can update its own tables too.That is, the spacecraft itself could update the star tables for many cycles by keeping track of changes to individual stars. It would need AI though to know how stable certain changes are, and drop or suspend tracking of stars, galaxies, or other objects that are unreliable to use either due to their intrinsic nature or because they are being obscured or dimmed because of a dust cloud or som

How do we know a pulsar's period cannot change over millenia? I mean all sorts of things can change a pulsars period.. collision with a red dwarf for one.

The periods of pulsars do increase, in a well-measured way, as they age. (Collision with another random star is so nonsensically rare that I doubt it has happened to a single pulsar in the history of our galaxy.) The periods of regular stars also change, but in less well-measured ways, because we know their periods so much less precisely. We can measure the time of arrival of a pulsar pulse to accuracy of less than a microsecond, which gives us a positional accuracy (timing error times the speed of light

I think using normal stars would be much more difficult, if you're talking about navigation beyond the solar system. Things like brightness will change considerably as you get closer, and motion will appear to increase too. I think measuring the pulse rate would provide the easiest means to identify a given star. Doing this for a bunch of them and fitting it to a known table would be relatively straightforward. I wouldn't be surprised if you could even figure out the current date and time based on the p

Should't we know the exact distance to the pulsar, in order to account for relativistic effects? or we just do bo care about the pulsar's actual position and we only care about the light from the pulsar coming to us?

GPS on Earth takes into account relativity in order to have a good precision, because satellites above Earth are in a different reference frame.